Tue. Mar 28th, 2023

Tau in person's hand on brown background.

Adrian Kingsley-Hughes/ZDNET

What is Tau?

Made by Rolling Square — the company behind the really cool inCharge X, 6-in-1 charging cable — Tau is designed to keep your smartphone charged up and your keys in one place. 

Tau is a tiny power bank key fob that magnetically attaches to a wall-mounted charger. In the morning, you grab your keys along with a fully-charged power bank. When you come back in the evening, you pop your keys back in the holder and charge up the power bank.

This way, you know where your keys are, and you always have a charged-up power bank.

Also: The 5 best portable power banks

Oh, and the power bank features a built-in USB-C, a Lightning cable, and a microUSB cable, so you don’t even have to remember to bring separate cables. 

Rolling Square Tau tech specs


  • Lightning (iPhone, iPad, iPod, AirPods)
  • USB-C devices
  • microUSB devices


  • Size: 60 x 45 x 12 mm / 2.32 x 1.77 x 0.47 in
  • Weight: 40 g / 1.4 oz
  • Capacity: 1400 mAh
  • Weight: 350 g / 12 oz

In the box

  • Tau power bank
  • Tau Dock and USB power cable
  • Screws and wall plugs
  • Nano suction double-sided layer
  • User manual
  • Eco-friendly packaging

Also: This tiny USB-C charger packs a lot of power

Tau package on a brown background.

Adrian Kingsley-Hughes/ZDNET

According to Rolling Square, Tau is the smallest power bank in the world. I can’t confirm this, but it certainly is small and light, and it’s no bigger than a car key fob. 

The 1400mAh battery isn’t going to be enough to fully charge a modern smartphone, and the 1A charging power isn’t going to be fast, but it will get you out of a low battery jam, for sure.

Model, input, output, and capacity info for Tau

Adrian Kingsley-Hughes/ZDNET

On the bottom of the key fob is a magnetic charging port that hooks in with the dock. The dock itself can be attached to a wall using the provided screws of a nano suction pad.

Closeup of Tau's magnetic charging port

Adrian Kingsley-Hughes/ZDNET

On the other end are the two charging cables, one for USB-C, the other for Lightning and microUSB.

Closeup of Tau's charging cables

Adrian Kingsley-Hughes/ZDNET

Also: Fake Lightning cables can damage your iPhone. Here’s how to make sure yours is genuine

Here’s the fob in the charging dock, with the dock charging cable attached. It’s a neat package.

Tau against a brown background in dock with keys attached

Tau in dock with keys attached… ready for action.

Adrian Kingsley-Hughes/ZDNET

Tau is everything I expect from Rolling Square — a quality product that actually solves a real problem. A tiny power bank disguised as a key fob and a well-mounted charging dock turns this from what might be considered a gimmick into a useful tool. 

This is perfect for people who forget where they’ve put their keys and run out of the house in the morning with a smartphone that’s almost dead.

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